1. Field of the Invention
The present invention relates to a sheet stacking apparatus for stacking on stack trays sheets with images formed thereon.
2. Related Background Art
A sheet post-handling apparatus has been widely used in which images are formed (or printed) on sheets by an image forming apparatus such as a copier and a printer; then, the sheets are stacked one over the other, sorted, and bound; then the printed sheets and bound sheets are stacked on a plurality of stack trays. With this type of conventional sheet post-handling apparatus, when a sheet surface sensor detects the position of a paper-surface in the lower tray by turn-off thereof the lower tray is caused to stop. The position at which the lower tray stops is a standby position. The above-mentioned configuration is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-53308 and Japanese Patent Application Laid-Open No. 2003-48661.
However, with the above-mentioned conventional sheet post-handling apparatus, friction between the walls of the stack tray and side edges of the sheets may cause the sheets to lean against the stacker walls. Also, the side edges of the sheets may be caught by a sensor flag projecting from the stacker wall and cause the sensor flag to remain ON, so that the lower tray descends much lower than a right sheet position. If the descending lower tray is forcibly caused to stop at the right sheet position, the sheets that are leaning against the stacker walls and/or are being caught by the sensor may drop from the tray due to vibration and reaction that occur upon stoppage of the tray.
Also, when chattering occur during a sheet-surface sensor generating OFF-output, or when the user places a stack of sheets on the tray during the tray descending, the tray is stopped at a position much lower than the right position. Further, when the user forcibly sets the sheet surface sensor to an ON-output, the tray continues to descend by a time length during which the sensor is ON.
With the aforementioned conventional method, the descending movement of a tray is stopped upon the OFF output of the sensor detector, requiring a longer time than necessary before the tray takes up a standby position.